This proposal focuses on the application of pericyclic reactions in new methodology for the synthesis of several important classes of carbocyclic and heterocyclic organic compounds. Part I describes the development of new strategies for the synthesis of five- and sixmembered aromatic, heteroaromatic, and dihydroaromatic systems. Intramolecular (4+2) cycloaddition reactions of conjugated enynes will be investigated and employed in new routes to highly condensed polycyclic systems. Heterocyclic variants of this process will be developed leading to five- and six-membered nitrogen and oxygen heterocycles. The application of this chemistry to the synthesis of furanophenalenone diterpenes, pentacyclic polyketide marine natural products with protein tyrosine kinase inhibitory activity, and to the synthesis of the antiviral agent nothapodytine B (mappicine ketone) will also be examined. Part II of the proposal describes the development of new methods for the synthesis of conjugated, unsaturated ketenes and their use in new annulation strategies for the synthesis of five-, six-, and seven-membered carbocyclic and heterocyclic compounds. Part III of the proposal describes the application of a tandem propargylic rearrangement-asymmetric inverse electron-demand intramolecular vinylallene Diels-Alder strategy in an exceptionally concise approach to the total synthesis of the nematode hatching stimulus agent glycinoeclepin A.